BrunoF
Your question is not clear ? What is it you wish to know ?
Reserve fuel is normally a combination of 2 (or possibly more ) variables. One is the fuel required to the alternate airport and the other is a fixed holding reserve ( often 30 minutes). There may be other factors involved that mean the figure is nearly always going to differ from one flight to the next, certainly from one aircraft type to the next.
I supect this is a flight sim' type question but perhaps you could clarify what it is you actually want to know ?